-Refactor JsonSerializer.Binder to not create wrapper when unneeded

Src/Newtonsoft.Json.Tests/Serialization/JsonSerializerTest.cs

@@ -1336,7 +1336,7 @@ namespace Newtonsoft.Json.Tests.Serialization
             Assert.AreEqual(customBinder, serializer.Binder);
 #pragma warning restore CS0618 // Type or member is obsolete
 
-            Assert.IsInstanceOf(typeof(SerializationBinderAdapter), serializer.SerializationBinder);
+            Assert.IsInstanceOf(typeof(DefaultSerializationBinder), serializer.SerializationBinder);
 
             serializer.SerializationBinder = customBinder;
             Assert.AreEqual(customBinder, serializer.SerializationBinder);
@@ -1570,7 +1570,7 @@ namespace Newtonsoft.Json.Tests.Serialization
             Assert.AreEqual(customBinder, serializerProxy.Binder);
 #pragma warning restore CS0618 // Type or member is obsolete
 
-            Assert.IsInstanceOf(typeof(SerializationBinderAdapter), serializerProxy.SerializationBinder);
+            Assert.IsInstanceOf(typeof(DefaultSerializationBinder), serializerProxy.SerializationBinder);
 
             serializerProxy.SerializationBinder = customBinder;
             Assert.AreEqual(customBinder, serializerProxy.SerializationBinder);

Src/Newtonsoft.Json.Tests/Serialization/TypeNameHandlingTests.cs

@@ -2151,6 +2151,21 @@ namespace Newtonsoft.Json.Tests.Serialization
             Assert.IsInstanceOf(typeof(FancyBinder), serializer.SerializationBinder);
         }
 
+        [Test]
+        public void SetOldBinderAndSerializationBinderReturnsWrapper()
+        {
+            var serializer = JsonSerializer.Create(new JsonSerializerSettings() { Binder = new OldBinder() });
+
+#pragma warning disable CS0618 // Type or member is obsolete
+            Assert.IsInstanceOf(typeof(OldBinder), serializer.Binder);
+#pragma warning restore CS0618 // Type or member is obsolete
+
+            var binder = serializer.SerializationBinder;
+
+            Assert.IsInstanceOf(typeof(SerializationBinderAdapter), binder);
+            Assert.AreEqual(typeof(string), binder.BindToType(null, null));
+        }
+
         public class FancyBinder : ISerializationBinder
         {
             private static readonly string Annotate = new string(':', 3);
@@ -2166,6 +2181,14 @@ namespace Newtonsoft.Json.Tests.Serialization
                 return null;
             }
         }
+
+        public class OldBinder : SerializationBinder
+        {
+            public override Type BindToType(string assemblyName, string typeName)
+            {
+                return typeof(string);
+            }
+        }
 #endif
     }
 

Src/Newtonsoft.Json/JsonSerializer.cs

@@ -111,8 +111,7 @@ namespace Newtonsoft.Json
                     return null;
                 }
 
-                SerializationBinder legacySerializationBinder = _serializationBinder as SerializationBinder;
-                if (legacySerializationBinder != null)
+                if (_serializationBinder is SerializationBinder legacySerializationBinder)
                 {
                     return legacySerializationBinder;
                 }
@@ -132,7 +131,7 @@ namespace Newtonsoft.Json
                     throw new ArgumentNullException(nameof(value), "Serialization binder cannot be null.");
                 }
 
-                _serializationBinder = new SerializationBinderAdapter(value);
+                _serializationBinder = value as ISerializationBinder ?? new SerializationBinderAdapter(value);
             }
         }